Intervertebral implant comprising a three-part articulation

ABSTRACT

An intervertebral implant including: a central axis, upper and lower terminal parts and a convex joint element. Each of the terminal parts come to rest against the end surfaces of two adjacent vertebras and opposite first and second concave inner surfaces, respectively. The convex joint element is situated between the terminal parts and rests in a sliding manner against the first and second concave inner surfaces of the two terminal parts. The, first concave surface is a partial surface of a first rotationally symmetrical external surface about the axis of rotation transverse to the central axis. The second concave inner surface of a second rotationally symmetrical conical external surface about the axis of rotation transverse to the central axis.

The present invention relates to an intervertebral implant defined inthe preamble of claim 1.

Upon removal of a natural, damaged intervertebral disk or a damagednucleus of an intervertebral disk, implants or prostheses are insertedinto the intervertebral space between two adjacent vertebras. Theobjective is also to restore the most natural conditions possible, thatis, in particular to restore the original intervertebral disk height andthence the original spacing between the two adjacent vertebras. Moreoverrelative displacements between the adjacent vertebras should be possiblein a natural way in the most hamper-free way possible. Preservation ofdisplaceability during forward/backward motions, that is when thevertebras shall be flexed or extended, also when the vertebras shall bebent, within natural boundaries, is then essential. Again the mutualrotations of the adjacent vertebras also should be possible withinnatural bounds.

A vertebral disk endosprosthesis of this kind is known from the BÜTTNEREuropean patent document 0 176 728. This known vertebral diskendoprosthesis substantially consists of two symmetrical, concavesealing plates of which the external surfaces each may be made to restagainst one of the end plates of the adjacent vertebras, and of a convexspacer configured between the mutually opposite concave sides of theterminal plates. This known vertebral disk endoprosthesis incurs thefollowing drawbacks:

-   -   the axes of rotation relating to bending/stretching and also to        laterally bending the adjacent vertebras by means of the        joint-like connection are defined only within a given range and        do not coincide with natural physiological conditions,    -   the play in displacement is the same laterally and        ventrally/dorsally because the prosthesis exhibits symmetry of        rotation when using the Pe-inlay. The natural averages of        motional play, that is the rotation of the terminal plates about        axes or the rotation transverse to the longitudinal axis of the        spinal column are 10°, 5° for stretching and 7° for lateral        motions;    -   the prosthesis does not include displacement restricting means        regarding relative rotation of the terminal plates about an axis        of rotation which is coaxial with or parallel to the        longitudinal axis of the spinal column (vertebra twisting), and    -   the prosthesis offers only minor damping against applied shocks.

The objective of the present invention is palliation. Its goal is tocreate an intervertebral implant offering a particular defined axis ofrotation for the lateral displacement of the spinal column, and for thebending and stretching of the adjacent vertebras, said axes of rotationcrossing each other and subtending different angles with the centralaxis.

The present invention solves the stated problem by means of anintervertebral implant and in particular by an intervertebral implantexhibiting the features of claim 1.

The intervertebral implant of the present invention comprises a centralaxis substantially parallel to or coaxial with the spinal column'slongitudinal axis, further a lower and an upper terminal part, eachcomprising an outermost surface transverse to the central axis and eachcomprising two mutually opposite surfaces, each surface being concave,further a joint element positioned in sliding manner at the concaveinner surfaces. The outermost surfaces of the terminal parts each may bemade to rest against one of the two end surfaces of two adjacentvertebras. The first concave inner surface configured at the upperterminal part is designed in the form of a first lateral surface whichis rotationally symmetric relative to the axis of rotation and runstransversely to the central axis. The second inner surface configured atthe lower terminal part is designed as a partial surface of a conicalouter surface which is rotationally symmetric to the axis of rotationthat is transverse to the central axis, as a result of which the secondaxis of rotation coincides with the cone's longitudinal axis.

It is assumed herebelow that the lower terminal part is affixed inposition and in that the joint element as well as the upper terminalpart are displaceable. Hence the second axis of rotation—which isdetermined by the concave second inner surface that is rotationallysymmetric relative to the second axis of rotation, also is affixed inposition, whereas the first axis of rotation, which is determined by thefirst concave inner surface that is rotationally symmetric relative tothe first axis of rotation, and hence is fixed in position relative tothe upper terminal part, together with the joint element and the upperterminal part shall be rotated about the second axis of rotation whenthe joint element together with the upper terminal part will be rotatedabout the second axis of rotation. Herebelow, the central axis also isassumed stationary relative to the lower terminal part.

In the preferred embodiment mode of the intervertebral implant of thepresent invention, the said axes of rotation will cross one another.Preferably these axes of rotation intersect the central axis.

Essentially the advantages offered by the present invention are deemedto be that its intervertebral implant enables the two axes of rotationto be configured in two different planes transverse to the central axis.As a result, the axis of rotation of central spinal column bending andthe central axis for spinal column bending/stretching may be offset fromeach other and may be configured within non-parallel planes. This designallows precisely imitating the positions of the natural axes ofrotation, and consequently the bio-mechanics of the two adjacentvertebras may substantially approximate the healthy condition of thespinal column, or even this healthy condition may be restoredcompletely. Because the physiological axes of rotation are taken intoaccount, no increased torques are applied to the ligaments, sinews andmuscles while retaining the distances between center of rotation andforce application point.

In another embodiment mode of the intervertebral implant of the presentinvention, the slide surfaces of the joint element are not complementaryto the concave inner surfaces of the terminal parts, resulting therebyin a point-like or linear contact between the slide surfaces and theconcave inner surfaces of the two terminal parts. Illustratively alinear contact may be attained in that the slide surfaces exhibit alesser radius of curvature at the joint element than do the adjoiningconcave inner surfaces of the terminal parts. Illustratively apoint-like contact between the first concave inner surface and theadjoining joint element at the slide surface may be attained by aspherical, ellipsoid of rotation or barrel-like design of the jointelement's slide surface and, depending on the embodiment mode, also ofthe first concave inner surface.

In yet another embodiment mode of the intervertebral implant of thepresent invention, the joint element comprises at least one convex slidesurface which is transverse to the central axis and complementary to theconcave inner surface of the adjoining terminal part. Preferably bothslide surfaces are complementary to the terminal parts' convex innersurfaces, the first slide surface being complementary to the concaveinner surface of the upper terminal part and the second slide surfacebeing complementary to the concave inner surface of the lower terminalpart. In this embodiment mode, the first concave inner surface and thefirst slide surface are designed as partial surfaces of a first externalsurface exhibiting symmetry of rotation and transverse to the centralaxis and they constitute the slide surfaces of a first joint rotatableabout the first axis of rotation. The second concave inner surface andthe second slide surface are designed as partial surfaces of anexternal, conical surface constituting the slide surfaces of a secondjoint rotatable about a second axis of rotation, said second axis ofrotation corresponding to the cone's longitudinal axis.

As regards a further embodiment mode of the intervertebral implant ofthe present invention, the two axes of rotation are configured in amanner that the second axis of rotation is contained in a plane alsocontaining the central axis and being traversed by the first axis ofrotation. This feature offers the advantage that the first axis ofrotation, and thereby the axis of rotation of spinal column bending andstretching shall be perpendicular to the vertebral implant's centralaxis when the joint element is not rotated about the second axis ofrotation. The second axis of rotation is used to laterally bend thespinal column and may include an angle α to the central axis matchingnatural displacement. Preferably this angle α is within the range of 60to 88°. By selecting the angle α and depending on the height of theintervertebral disk, the different lumbar spinal column segments may beimitated physiologically.

In yet another embodiment mode of the intervertebral implant of thepresent invention, the external surface rotationally symmetrical to thefirst axis of rotation is a circularly cylindrical external surface. Thefirst convex inner surface at one of the terminal parts as well as thefirst slide surface at the joint element therefore are designed aspartial surfaces of a circularly cylindrical external surface, wherebythe first joint is rotatable only relative to the first axis ofrotation. As a result, a specific bending/stretching motion is madepossible. Moreover separate consideration of this scenario of motion isfeasible because the axis of rotation of lateral bending is situatedelsewhere.

Instead of the external surface rotationally symmetrical relative to thefirst axis of rotation being a circularly cylindrical external surface,it may also be in the form of an external conical surface. Furtherdesigns of the external surface rotationally symmetrical with respect tothe first axis of rotation illustratively are surface portions of anellipsoid of revolution, a double cone or also another arbitrary body ofrevolution.

In yet another embodiment mode of the intervertebral implant of thepresent invention, the first axis of rotation intersects the centralaxis and as a result, when the spinal column is bent or stretched, thecenter of rotation of the two vertebras adjoining the intervertebralimplant will be situated on the intervertebral implant's central axis.The second axis of rotation of the second joint also intersects thecentral axis. In this case the two axes of rotation are apart by aminimum distance A. Preferably this distance A is between 0 and 18 mm.This distance A depends on the anatomical particulars of the centers ofrotation, for instance on the fact that the axis of rotation of lateralbending drops diagonally in the dorsal direction in the median plane ofthe human body.

In yet another embodiment mode of the intervertebral implant of thepresent invention, the outermost surfaces of the terminal parts exhibita three-dimensional structure, for instance a roughened surface, toenhance the growth of the adjoining vertebras onto the intervertebralimplant.

As regards another embodiment mode of the intervertebral implant of thepresent invention, the three-dimensional structure is replaced by agrid, preferably a titanium grid, again to enhance growth by thevertebra end plates onto the intervertebral implant.

In still another embodiment mode of the intervertebral implant of thepresent invention, at least one of the terminal parts comprises firstand second stops to limit the mutual motions between the terminal parts.These stops serve to limit the relative motion of the terminal partsabout the first axis of rotation enabling bending and stretching theadjacent vertebras. Preferably lateral bending of the two vertebrasadjoining the intervertebral implant shall be restricted by third stops.These third stops limit the relative rotation between the terminal partsabout the second axis of rotation. The stops are designed in a manner toallow maximum stretching between 2 and 15° and maximum lateral bendingbetween ±5° and ±10°.

As regards one embodiment mode of the intervertebral implant of thepresent invention, one of the terminal parts includes three elements.This terminal part comprises an outermost cover plate, further a jointpan enclosing the concave inner surface and an elastically deformingspacer mounted in-between transversely to the central axis. Insertingsaid elastically deforming spacer offers the advantage that on one handcompressive motion of the two adjacent vertebras shall be damped and onthe other hand that shearing and torsional displacements between the twoterminal parts of the intervertebral implant are feasible.

Further advantageous embodiments of the present invention arecharacterized in the dependent claims.

The invention and further of its developments are elucidated below inthe partly schematic drawings of several illustrative embodiments.

FIG. 1 is an exploded view of an embodiment mode of the intervertebralimplant of the invention,

FIG. 2 is a perspective view of the joint element with the externalsurfaces containing the slide surfaces and being rotationallysymmetrical to the axes of rotation of the embodiment mode shown in FIG.1 of the intervertebral implant of the invention,

FIG. 3 is a sideview of an embodiment mode of the intervertebral implantof the invention,

FIG. 4 is a view from the rear of the embodiment mode shown in FIG. 3 ofthe intervertebral implant of the invention, and

FIG. 5 is an exploded view of another embodiment mode of theintervertebral implant of the invention.

FIG. 1 shows an embodiment mode of the intervertebral implant of theinvention comprising an upper and a lower terminal part 2; 3intersecting the central axis 1 and furthermore a joint element 4 whichis situated between the terminal parts 2; 3 and which also intersectsthe central axis 1. Each terminal part 2; 3 comprises an outermostsurface 5; 6 matched to the end plates of the adjoining vertebras andrunning transversely to the central axis 1, each of said outermostsurfaces being displaceable in a manner to come to rest against one ofthe mutually end surfaces of two adjacent vertebras. The mutuallyopposite two terminal parts 2; 3 each comprise one concave inner surface7; 8 whereas the joint element 4 comprises two convex slide surfaces 9;10 of which the slide surface 9 is complementary to the concave innersurface 7 of the upper terminal part 2 and the other convex slidesurface 10 is complementary to the concave inner surface 8 of the lowerterminal part 3. The first concave inner surface 7 and the first slidesurface 9 complementary thereto constitute the slide surfaces of a firstjoint rotatable about the first axis of rotation 12 between the jointelement 4 and the upper terminal part 2. When the joint element 4 is inits initial position, that is, when it was not rotated about the secondaxis of rotation 14, the first axis of rotation 12 will be perpendicularto the central axis 1. Rotation of the upper terminal part 2 about thefirst axis of rotation 12 entails a bending or stretching displacementof the vertebras adjoining the terminal parts 2; 3. The first concaveinner surface 7 at the upper terminal part 2 and its complementary firstslide surface 9 at the joint element 4 are designed in the embodimentmode presently being discussed of the intervertebral implant of theinvention as partial surfaces of a circularly cylindrical outermostsurface rotationally symmetrical to the axis of rotation 12. The secondconcave inner surface 8 and its complementary second slide surface 10constitute the slide surfaces of a second joint rotatable about a secondaxis of rotation 14 and situated between the joint element 4 and thelower terminal part 3. This second axis of rotation 14 intersects thecentral axis 1 at an angle α but does not intersect the axis of rotation12. As shown in FIG. 2, the second axis of rotation 14 is situated in aplane 17 containing the central axis 1 and is traversed by the firstaxis of rotation 12. Accordingly rotating the joint element 4 togetherwith the upper terminal part 2 about the second axis of rotation 14allows laterally bending the two vertebras adjoining the intervertebralimplant. FIG. 2 also shows that the external surface 11 rotationallysymmetrical about the first axis of rotation 12 is a circularlycylindrical external surface of which the first slide surface 9constitutes a surface patch. The second slide surface 10 is a surfacepatch of a second circularly cylindrical external surface 16 which isrotationally symmetrical with respect to the second axis of rotation 14.Moreover the two axes of rotation are a distance A apart. As shown inFIG. 2, as the joint element 4 is rotated about the second axis ofrotation, the first axis of rotation 12 moves along an arc 13 of radiusA relative to and concentrically with the point of intersection 15between the central axis 1 and the second axis of rotation 14.

FIG. 3 shows an embodiment mode of the intervertebral implant of theinvention that differs from that of FIG. 1 in that the upper terminalpart 2 consists of three elements. The upper terminal part 2 comprisesin axially outermost manner the upper cover plate 24 containing theoutermost surface 5, further a joint pan 26 pointing toward the jointelement 4 and an elastically deforming spacer 25 between the joint pan26 and the cover plate 24. The intervertebral implant moreover comprisesa front side 19 pointing toward the cone tip 18, and oppositely a rearside 20. The intervertebral implant is designed in a manner thatfollowing implantation, the front side 19 shall be positioned to therear of the intervertebral space. As a result the second axis ofrotation 14 runs from the front to the rear. The rotation of the upperterminal part 2 about the first axis of rotation 12 (FIG. 1)perpendicular to the plane of the Figure is limited by the two stops 21;22. The first stop 21—which serves to limit a rotation of the twoterminal parts 2; 3 relative to each other shortening the front side 19parallel to the central axis 4 and hence restricting the stretchingdisplacement of the two adjoining vertebras—is mounted at the front side19 on the upper terminal part 2, whereas the second stop 22 serves torestrict a rotation of the two terminal parts 2; 3 relative to eachother, shortening the rear side 20 parallel to the central axis 1,restricting the stretching displacement of the two adjoining vertebras.The first and second stops 21; 22 are constituted by the lower ends 32;33 respectively situated on the front side 19 and rear side 20 of theupper terminal part 2 that rest against the inner surface 31 facing thejoint element 4, of the lower terminal part 3, after the maximumadmissible angle of rotation has been reached. FIG. 4 shows that thelower end 33 is rounded off at the rear side 20 (FIG. 3), the center ofthis rounded segment coinciding with the intersection between the secondaxis of rotation 14 (FIG. 3) and the rear side 20 of the intervertebralimplant, as a result of which the first stop 22—when the upper terminalpart 2 is rotated about the second axis of rotation 14 (FIG. 3)—shallcome to rest at the same angle of rotation about the first axis ofrotation 12 against the inner surface 31 of the base plate 30 of thelower terminal part 3. Furthermore the second slide surface 10 (FIG. 1)is offset at the rear side 20 of the intervertebral implant over part ofits periphery and part of its length. The second concave inner surface 8is similarly offset. The stop 23 ensuing from this offset limits theangle of rotation of the upper terminal part 2 about the second axis ofrotation 14, as a result of which the lateral bending of the twovertebras adjoining the intervertebral implant shall be restricted. Thestops 21; 22; 23 are designed in a manner to allow bending the adjacentvertebras through an angle β of 10°, stretching by an angle γ=5° andlateral bending by an angle of δ=±7°.

The embodiment mode of the intervertebral implant shown in FIG. 5differs from the embodiment modes of FIG. 1, 3 or 4 only in that grids27 are present externally on the cover plate 24 and also on the baseplate 30 and in that the first convex slide surface 9 comprises anelevation 28 concentric with the first axis of rotation 12 and that thefirst concave inner surface 7 contains a recess 29, the elevation 28being tangentially displaced in the recess 29 when the upper terminalpart 2 is rotated relative to the joint element 4. The elevation 28 runsparallel to the first axis of rotation 12 only over a portion of thefirst slide surface 9 whereas the peripheral extension of the elevation28 runs over the entire first slide surface 9. The recess 29 in thefirst concave inner surface 29 is complementary to the said elevation.Other embodiment modes of the intervertebral implant of the inventionalso allow elevations 28 which are peripherally present only over aportion of the first slide surface 9. The elevation 28 engaging therecess 29 offers the advantages that on one hand lateral stabilizationbetween the first terminal part 2 and the joint element 4 is feasible,shearing motion of the adjoining vertebras parallel to the first axis ofrotation 12 being precluded, and on the other hand that it is possibleto center the joint element 4 within the intervertebral implant, as aresult of which the joint element 4 cannot be displaced parallel to thefirst axis of rotation relative to the first terminal part 2. Preferablythe grids 27 are titanium grids that also may be curved so that optimalgrowth of the adjoining vertebras onto the intervertebral implant shallbe possible.

1. An intervertebral implant having a central axis substantially parallel to or coaxial with an axis of a spinal column, comprising: an upper and a lower terminal part each fitted with an outermost surface configured transversely to the central axis, said upper terminal part having a first curved inner surface and said lower terminal part having a second curved inner surface, said first and second curved surfaces being opposite one another; and a joint element configured between the terminal parts and resting in a sliding manner against the curved inner surfaces of the upper and lower terminal parts, the joint element including first and second external convex slide surfaces, the first slide surface contacting the first curved inner surface of the upper terminal part, the second slide surface contacting the second curved inner surface of the lower terminal part, the first curved inner surface and the first slide surface forming a first joint rotatable about a first axis of rotation, the first axis of rotation being perpendicular to the central axis when in an initial position, the second curved inner surface and the second slide surface forming a second joint rotatable about a second axis of rotation, the second axis of rotation intersecting the central axis at an acute angle α, the second axis of rotation being spaced apart from the first axis of rotation by a distance A as measured along the central axis, wherein 0<distance A<18 mm.
 2. The intervertebral implant as claimed in claim 1, wherein the first axis of rotation and the second axis of rotation cross each other.
 3. The intervertebral implant as claimed in claim 1, wherein the first curved inner surface has a first radius of curvature and the first external convex slide surface has a second radius of curvature, the first radius of curvature being not equal to the second radius of curvature.
 4. The intervertebral implant as claimed in claim 1, wherein at least one of the slide surfaces has a first radius of curvature and at least one of the curved inner surfaces of the terminal parts has a second radius of curvature, the first radius of curvature being not equal to the second radius of curvature.
 5. The intervertebral implant as claimed in claim 1, wherein the first slide surface of the joint element is complementary to the first curved inner surface.
 6. The intervertebral implant as claimed in claim 1, wherein the second slide surface of the joint element is complementary to the second curved inner surface.
 7. The intervertebral implant as claimed in claim 1, wherein the angle α is between 60 and 88 degrees.
 8. The intervertebral implant as claimed in claim 1, wherein the outermost surfaces exhibit a three-dimensional structure.
 9. The intervertebral implant as claimed in claim 1, wherein the outermost surfaces are titanium grids that can be connected to the terminal parts.
 10. The intervertebral implant as claimed in claim 1, wherein: at least one of the terminal parts comprises a first rotation-restricting stop shortening a front side of the intervertebral implant parallel to the central axis about the first axis of rotation at an angle of rotation γ between 5 and 15 degrees; and at least one of the terminal parts includes a second rotation-restricting stop shortening a rear side of the intervertebral implant parallel to the central axis about the first axis of rotation at an angle of rotation β between 2 and 15 degrees.
 11. The intervertebral implant as claimed in claim 10, further comprising a third rotation-restricting stop restricting the rotation about the second axis of rotation at a maximum angle of rotation δ between −0.5 degrees and +10 degrees.
 12. The intervertebral implant as claimed in claim 1, wherein at least one of the terminal parts is a three-element part and comprises an outermost cover plate, a joint pan enclosing the curved inner surface and in-between an elastically deforming spacer. 